The automotive industry has for many years provide vehicles with electronically controlled elements. For example, the elements may be an airbag for deployment in the event of an accident or crash. The crash is detected typically in about 10-15 milliseconds. Then an electric signal is sent to fire the airbag. This is typically accomplished by means of squib which ignites a propellant (e.g. sodium azide) which generates a gas (e.g. nitrogen) which in turn inflates the airbag. This stage typically takes about 45-55 milliseconds and then within a further 75-80 milliseconds the airbag deflates. A squib is a small explosive device or other firing device for a safety device or other type of device: for example, an airbag activating component.
The energy required to fire the squib is generally provided by a reservoir capacitor. This capacitor must store as much energy as possible and as such the voltages is typically high (˜35V). An example of a current squib driver circuit has an output driver comprising two switches (a high-side and a low-side). The high-side is driven in a current limitation mode which generates a very high energy requirement for the power MOSFET. For example 35V and 2.1 A for a 1 ms period would equate to 73.5 mJ. As the MOSFET size is related to the amount of energy to be absorbed, the higher the energy the larger the MOSFET. This can have a large impact on the size and cost of the ASIIC.
Airbag applications also call for a safing switch in order to increase the overall safety by adding redundancy. This is achieved by providing a power switch that is off until a redundant microchip allows the firing of the squib. The redundant chip and squib are in series. These guarantees that the squib does not fire the airbag until all crash algorithms and validation are completed to confirm the need for airbag deployment. The safing switch is generally an external MOSFET device 110.
One object of the present invention is to overcome at least some of the disadvantages of the prior art. A further object of the present invention is to reduce the size of components in a driving circuit.